It’s the first rule of “Fight Club” – you don’t talk about it. Unfortunately for executives at 20th Century Fox, audiences are taking that order to heart.

Thanks to poor word-of-

mouth and an ultra violent content, the pummel fest was for all practical purposes KO’d at the box office in its second weekend in the ring, with earnings plummeting 43 percent.

On its opening weekend, the Brad Pitt starrer barely scraped into the No. 1 position as it took in $11 million. Fox studio execs were counting on strong buzz among film fans to build audiences.

But “Fight Club” showed little staying power and was bested by “The Best Man” and “Double Jeopardy,” dropping to No. 3 with a puny $6.3 million box-office haul.

Its total take now stands at about $21 million.

Theories abound as to why one of the year’s most controversial movies has not connected with audiences, but movie critic Leonard Maltin was most blunt.

“The main reason it sank was because it wasn’t very good,” he says. “Unquestionably, the violence puts off some people, but I didn’t have trouble with the violence. I had trouble with the story – it made no sense.”

Maltin says a surprise-

ending device, which worked so well in generating repeat business for “The Sixth Sense,” failed dismally in “Club’s” case.

“‘The Sixth Sense’s’ ending so intrigues you that you’re curious to see it again to work out how the pieces fit together,” he says. “But ‘Fight Club’s ending makes no sense – it doesn’t inspire you to see it again and it made me sorry I’d sat through it the first time.”

David Fincher’s subversive and audacious film has polarized critics. Rolling Stone dubbed it “a masterpiece,” while others have accused it of irresponsibly advocating violent and anti-social behavior.

The bleak and bloody drama, about young men using bouts of bare-

knuckled fighting to combat their disenchantment with the world, targets males in their early 20s, and exit polls have shown women are finding it a big turn-off.

“It’s tough to target a film specifically to males because you’re obviously cutting out half of the possible audience,” said Rich Ingrassia, a movie industry analyst with Paul Kagan Associates.

“This was not marketed as a mainstream movie,” he said. “This was a dark and edgy film, and any film like that is not going to bring in every demographic to get that huge initial box office.”

However, the film’s grim theme alone cannot be blamed for its poor showing. After all, Fincher’s 1995 film “Seven,” about a serial killer who uses the seven deadly sins as his modus operandi, became something of a phenomenon, racking up more than $100 million in ticket sales.

“Dark subject matter seems to fill Fincher’s oeuvre,” said Tom Borys, president of A.C. Nielsen EDI, which does box-office tracking for the film industry. “But ‘Fight Club’ isn’t connecting as well as his previous work.”

“Seven” took in $13.9 million in its opening weekend and only dropped 11 percent the following weekend. Fincher’s next effort, “The Game,” opened at $14.3 million and dropped 34 percent.

“There has been a lot of attention given to violence in films and perhaps this will be singled out as the one marking a shift in general tastes away from harder action films,” Borys said.